Quark models and hadron spectroscopy

When I first entered the field, I was told that even the inventor of the idea of quarks, Murray Gell-Mann, considered them as useful mathematical abstractions rather than real physical objects. However, after the discovery of the J/psi in 1974, this rapidly changed. One of the lasting triumphs of particle physics soon after that was the understanding that all the strongly interacting particles are made of real, dynamical quarks. This goes beyond just the overall quantum numbers – many of the dynamical properties are manifestations of the quark substructure also.

My earliest work was focused on understanding quarks. Gene Golowich, Barry Holstein and I were enthusiasts for the MIT bag model, which had the advantage of using light relativistic quarks with small masses appropriate for QCD. Our first work explored the static properties of hadrons – I probably learned as much from this project as any future one. Here is a link to a later review of bag model work.

I later had the opportunity of being a postdoc at MIT where it was a pleasure to work with Ken Johnson. We tackled the issue of how to understand the pion and chiral relations in the bag model, and got some interesting and positive results.

Also with Ken Johnson, we got involved in the study of exotic states made of gluons – glueballs. The bag model predicted these in the middle of the resonance region where quark model excited states also exist. This lead to a interesting period studying hadron spectroscopy and interacting heavily with experimenters. This was fun and we did make some progress in understanding the hadron spectrum. However, progress eventually slowed, and a clear understanding of hadron spectroscopy is still lacking today.

One of the drawbacks of the quark model was the fact that it truly was a “model”. After years working on the subject it became clear that it was not a controlled approximation. By making apparently reasonable “improvements”, one could get very different results. When the use of more rigorous chiral effective field theory methods developed, I dropped the quark model work and moved in that direction.

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Research and teaching pages for John Donoghue